Tissue fibrosis can be caused by radiation, trauma, medications, or toxins, and fibrosis can occur anywhere in the body including the oral mucosa, lung, kidney, liver, and skin. One characteristic of fibrosis is an increase in the synthesis of collagen type I (COL1) and decreases in both synthesis of COL3 and degradation of COL1. The presence of contractile fibroblasts (FB), termed myofibroblasts (myoFB), is considered a hallmark of the fibrotic response. Another hallmark of fibrosis is increased cell death of hepatocytes, alveolar epithelial cells, and endothelial cells (ECs). The level of apoptosis and degree of fibrosis have been related in hepatic, pulmonary, and renal fibrosis, suggesting that apoptotic cells directly influence the development of fibrosis. The mechanisms that dictate the interaction of apoptotic cells and fibrosis are poorly understood. EC apoptosis is a prominent feature in the latter stages of oral and dermal wound healing, as apoptosis is requisite to the pruning of the robust vascular bed that is produced during wound healing. The presence of high levels of apoptotic ECs in the resolving wound suggests that endothelial-fibroblast interactions may play a role in scarring and fibrotic outcomes. In renal, hepatic, and bleomycin-induced fibrosis, potent paracrine signaling factors secreted by apoptotic ECs have been identified. These factors can act directly on FBs to induce proliferation, myoFB differentiation, and collagen synthesis. We hypothesize that apoptotic ECs contribute to fibrosis and scar formation by secreting factors that increase dermal myoFB differentiation, collagen synthesis, and FB proliferation, while decreasing collagen degradation in FBs. The current research will provide the first examination of the mechanism of interaction between apoptotic ECs and FBs in dermal fibrosis. Aim 1 will establish a role for apoptotic EC as mediators of fibrosis in vivo. In vivo, te effect of apoptotic ECs will be examined by monitoring myoFB differentiation, collagen synthesis, and FB proliferation following intradermal injection of apoptotic ECs or their secreted factors. Aim 2 will determine the mechanism of interaction between apoptotic ECs and FB function in vitro. Normal human dermal FBs will be exposed to conditioned media from apoptotic ECs. The fibrogenic response and downstream signaling will be examined. Throughout the experimental plan, the fibrogenic response will be assessed by determining the ratios of COL1:COL3 and MMP1:TIMP1, both known characteristics of a fibrotic reaction. The long term goal of the proposed research is to uncover the mechanisms that influence scar formation and fibrosis in skin. This project has the potential to have a significant impact on the understating o fibrosis in scar formation as well as other related conditions, such as renal, hepatic, pulmonary, and cardiac fibrosis. Elucidation of these mechanisms will identify opportunities for therapeutic intervention in fibrosis.